mips: dts: ralink: mt7621: rename to GnuBee GB-PC1 and GnuBee GB-PC2

Rename GB-PC1 to GnuBee GB-PC1, and GB-PC2 to GnuBee GB-PC2 to include
brand and model name.

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Acked-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>